Variable Frequency Starting Control Method Of AC Excited Motor On Rotor Side

Compared with traditional synchronous motors and asynchronous motors,AC-excited variable-speed units have irreplaceable advantages in practical applications,such as flexible and controllable,at the same time help to improve the stability of the power system.At present,AC excitation variable speed units,especially doubly-fed induction generators,are widely used in wind,hydropower and other green renewable energy power generation and flywheel energy storage systems,and various studies on them have gradually become popular.When the variable-speed pumped-storage unit is in pumping mode,since the outside does not provide motive power to the motor,it does not have the ability to start itself.Solving the above problem is the key foundation for the development and promotion of AC-excited variable-speed pumped storage units.Taking the pumping condition of the variable-speed pumped storage unit as the premise in this thesis,two different rotor-side variable frequency starting control methods are adopted,without using other auxiliary starting equipment,only the rotor-side converter of the AC excitation system is used,and the soft start of the AC excitation motor is realized.Firstly,this thesis introduces the basic principles of AC-excited motors.For the starting method on the rotor side with stator short-circuit,the changes of the stator and rotor magnetic fields during the starting process and the relationship between the magnetic fields and the rotor speed are analyzed.According to the principle of open-loop constant voltage-frequency ratio control,an open-loop starting model on the rotor side is established,and the starting processes of AC excited motors with three different parameters are simulated.Through comparative analysis,the results show that open-loop starting method has the limitations of large starting current and too long starting time for motors with large moments of inertia.Secondly,in view of the limitations of the above-mentioned starting method,with the goal of reducing the starting current and shortening the starting time,a closed-loop staged starting control method with stator short-circuit is determined.Through the segmented calculation of the stator flux reference value,the starting process is divided into:excitation stage,constant torque starting stage and field weakening starting stage.Among them,the torque current component is controlled to zero in the excitation phase to ensure a larger excitation current component,so that the motor quickly reaches the rated flux linkage,and the starting current does not exceed the rated range.The constant torque stage realizes the accelerated start of the motor.In the field weakening stage,under the premise of ensuring the performance of the AC excitation system,the torque current component is controlled to a larger value to achieve a steady increase in the speed of the unit,and finally the start is completed.Based on the analysis of the above-mentioned starting process,a simulation model of AC-excited pumped storage unit is built to verify the correctness of the control method,and the influence of the motor’s moment of inertia on its starting time is compared.Finally,in order to make the motor start faster,a control method of stator voltage reduction and grid connection and rotor side variable frequency starting is proposed.The power plant backup transformer is used to obtain a suitable stator side starting voltage,and the rotor side converter is used to control the torque current component and the excitation current component.On the basis of ensuring that the stator current basically does not exceed its rated current,this method greatly shortens the starting time compared with the closed-loop staged starting control method of the stator short circuit,but there are problems of current and electromagnetic torque fluctuations in the initial stage of starting.For this reason,the control method is further improved,that is,the full control device of the converter is not triggered in the first 5 seconds of starting,and only its parallel diode is used for starting.The feasibility of the above-mentioned control method is verified by simulating the starting process of the large-capacity unit.